In recent years governmental and other bodies have become increasingly active in resisting air pollution. Concentrations of SO.sub.2 in the atmosphere are one form of air pollution that has evoked much criticism. In the past, industrial facilities, such as electrical power plants, sulfuric acid plants, metal smelters, and petroleum refineries merely vented their flue gases containing sulfurous emissions to the atmosphere. The growth and concentration of industrial facilities emitting sulfur oxides gradually increased the ground level SO.sub.2 concentrations and airborne particulate sulfates derived from SO.sub.2 to levels considered to constitute harmful pollution.
Initially, the problem of reducing ground level SO.sub.2 concentrations was solved by high stacks which would more widely distribute the SO.sub.2, to maintain the SO.sub.2 concentration level below the polluting level. Further expansion of industry and an awareness of the mechanisms by which SO.sub.2 is converted to harmful sulfate particulate dispersions in the atmosphere have caused tightening of governmental-imposed SO.sub.2 pollution limits, which has forced both industry and government to seek processes for reducing or eliminating SO.sub.2 pollution.
A number of processes have been proposed for the purpose of reducing SO.sub.2 emissions, and have been successful to varying degrees. Initial commercial attempts generally fell into the category of "throwaway" processes. These processes are characterized by the conversion of SO.sub.2 to, for example, calcium sulfite or calcium sulfate, or a mixture thereof, that is discarded in a landfill. These approaches have helped in the area of air pollution, but do not result in elimination of other pollution, since the sulfite which has a high chemical oxygen demand can be leached and thus pollute ground waters. Recent attempts have therefore been directed to finding an economically viable method for recovering a useful product while avoiding SO.sub.2 or other pollution. The present invention provides such methods and also relates to converting the SO.sub.2 pollutant to essentially pure elemental sulfur of a quality useful to industry.
German Patent publication No. 2,001,284 discloses a process for treatment of waste gases from a sulfur recovery plant. Waste gas is contacted with an aqueous solution of an absorbent in which the water acts as the main catalyst for reaction of H.sub.2 S and SO.sub.2 to sulfur. A first solution is formed by reacting H.sub.2 S and an absorbent; the absorbent containing SO.sub.2 is then reacted to form a second solution. The first and second solutions are reacted to regenerate the absorbent and to release sulfur in a slurry, which is subjected to heating and melting to separate the sulfur, and the absorbent is recycled to the absorber reactor. This German publication teaches the use of conventional absorbents for H.sub.2 S in the process. Examples of the absorbents include K.sub.3 PO.sub.4, K.sub.2 CO.sub.3, and Na.sub.3 PO.sub.4 as well as methyl, diethyl, and triethyl amines. Each of these absorbents is highly alkaline, resulting in a high pH in the absorption zone; the use of K.sub.3 PO.sub.4, for example, may result in the formation of colloidal sulfur.
U.S. Pat. No. 2,031,802 describes a process wherein solutions of monobasic phosphates and sodium citrate are disclosed as suitable solutions for absorption of SO.sub.2 ; the SO.sub.2 -laden solution is regenerated by heating it to its boiling point to drive off the SO.sub.2. U.S. Pat. No. 2,368,595 discloses the use of tripotassium phosphate, a highly alkaline material, as an absorbent for SO.sub.2 and H.sub.2 S, followed by stripping of the SO.sub.2 and H.sub.2 S from the absorption media. U.S. Pat. No. 2,563,437 discloses absorbing SO.sub.2 in Al.sub.2 (SO.sub.4).sub.3 and H.sub.2 SO.sub.4 and then reacting the solution with H.sub.2 S to produce filterable sulfur, separating the sulfur, and recycling the supernatant liquid. U.S. Pat. No. 2,729,543 discloses the absorption of SO.sub.2 by salts of acids which can be buffered in the 4 to 6 pH range, specifically citing citric acid. The solution containing dissolved SO.sub.2 is contacted with H.sub.2 S to precipitate sulfur.
Another prior process is the Bureau of Mines process, described in detail in Bureau of Mines Report of Investigations/1973 Report RI 7774, discussed later herein.
All of the numerous processes described in the prior art have one or more grave disadvantages. These disadvantages include (a) loss of absorbent from the system, e.g., by volatilization, absorption on the sulfur product, or decomposition; (b) difficulty of, or consumption of large quantities of energy for, separating SO.sub.2 from the absorption medium; (c) failure to produce a satisfactory sulfur product for reuse in a captive process or for resale on the market; (d) production of undesirable waste materials which cause further pollution; or (e) requirements for separate equipment and treatment of the SO.sub.2 -laden gases to remove particulate material from the gases prior to treatment to remove SO.sub.2. Each of these disadvantages also results in undesirably high capital or operating costs or both, making installation of the abatement process impractical.
In all of the known systems wherein elemental sulfur is the desired product, the SO.sub.2 -laden gases are cleaned to remove particulate material prior to absorbing the SO.sub.2 therefrom. The instant invention does not require such prior cleaning of the SO.sub.2 laden gases, or the high equipment and operating costs for such cleaning, while producing a comparable sulfur product.
The principal object of the present invention is to provide a process for reducing SO.sub.2 in gaseous emissions to far below the present level in an economically practicable manner and without causing further pollution problems.